The human heart is a pump—a complex and critical pump. As with any pump, the heart can become clogged and wear out over time. When wear and damage to the heart become sufficiently serious, the owner of the heart is said to have suffered severe heart failure. In such a situation, it is often necessary for the person to receive mechanical assistance for the heart or to receive a heart transplant. Where the person receives a transplant, mechanical assistance may still be needed until a donor heart is available.
Blood pumps are commonly used to provide mechanical assistance to the left ventricle of the heart. The left ventricle pushes blood out through the aorta and into the person's body. The left ventricle bears about eighty percent of the heart's load, and therefore is generally the first part of the heart to require assistance.
Ventricular assistance may be provided by a pump that is implanted in a person's abdomen, and that is connected in parallel with the person's cardiovascular system. In particular, an inflow conduit for a pump may be attached to the tip of the left ventricle, and an outflow conduit may be attached to the wall of the aorta. In this manner, some blood may take its normal route out of the ventricle and into the aorta, and other blood may pass through the pump, receive a boost, and be pushed into the body via the aorta.
The speed of the pump, and in turn the level of assistance provided by the pump, generally must be carefully controlled. The pump should be able to adapt to changes in demand for blood. For example, when a person exercises or is otherwise stressed, the pump generally must run faster to ensure that the heart provides adequate blood to the body. In adapting to changes in blood demand, the pump cannot run so slowly that blood does not get out of the heart and into the body. Similarly, the pump should not run so fast that it causes suction in the left ventricle. When suction occurs, the assist pump receives less blood flow, and the contractile properties of the ventricle may be adversely affected as the ventricle begins to collapse. Nonetheless, a rotary pump is generally most effective when it is running at the upper end of its range, which may be near the speed that causes ventricular collapse. Thus, it is important that a suction event that signifies the onset of ventricular collapse be sensed so that the pump may operate at an optimum speed.
It is possible to use sensors external to a blood pump, such as pressure transducers, to measure the flow rate and pressure through a pump, so that the pump's speed may be adjusted to compensate for changes in blood requirements. However, external sensors add complexity to a blood pump system, and also add complexity to the surgical procedure used to implant the system. In addition, sensors are generally encapsulated or coated with biological materials, which can render them unfit for long-term use.
Blood pumps may also sense or detect the activity of the heart indirectly by monitoring the blood flow through the pump, such as by measuring the current draw and speed of the pump over time. Such monitored variables may then be used to compute a revised speed for the pump, such as when ventricular suction is detected. Several such methods for adjusting the speed of an implantable blood pump are disclosed in U.S. Pat. Nos. 5,888,242 and 6,066,086. For example, cyclical current fluctuations of the pump during systole-diastole may be monitored for a detectable current spike that is indicative of the onset of ventricular collapse by suction. In addition, the increase in flow rate lessens as pump speed is increased, so that the derivative of the flow rate with respect to speed can indicate the need to reduce the pump speed. Moreover, the second harmonic of the current fluctuation increases just before ventricular collapse, so that the harmonic may be monitored to help indicate an appropriate pump speed. Also, the opening and closing of the aortic and mitral valves may be monitored, either with implanted microphones/hydrophones or by measuring the pulsatility of the motor current, to indicate the onset of ventricular collapse.
As indicated, there is a need for apparatuses or methods that reliably control the speed of an implantable blood pump. In particular, there is a need to provide such control to avoid left ventricular collapse over a range of blood demand by a patient.